It might be the heat shield on the catalytic converter, i had one of the welds break loose and it vibrated only during idling, it might also be a loose hose clamp, i also had one of those knocking, or it might be a valve knocking, I know its not a piston cuz that would be continuously knocking, theres a number of things it could be unless you describe the sound more distinctly, also if it only does it when its idling why not try to find the source or at least the general area of the knock for us? I know many hondas has a "Honda Knock" thats normal, my prelude has one and it only knocks when cold and idling, as well as every other honda ive ever started and have heard start, and it is a very distinct sound